This application claims priority to International Application Number PCT/GB00/00124, filed Jan. 20, 2000 which for purposes of disclosure is incorporated herein by specific reference.
1. The Field of the Invention
This invention relates to a screening device for separating-out material in a predetermined size range from a supply of bulk material to the screening device.
2. Relevant Technology
The material to be screened can take many forms, including site clearance material, and crushed stone, and the device has screening apertures which allow material smaller in size than the apertures to pass through while leaving residual material in the device (greater in size than the apertures) to be discharged separately. Depending upon requirements, desired material separated-out from the bulk material supply may comprise the material which has passed through the screening apertures, and/or may comprise the material of greater size than the screening apertures which is discharged separately.
In the screening of crushed stone, or site clearance material (including top soil), it is known to provide so-called xe2x80x9cscreen decksxe2x80x9d which have a generally planar screening surface which is gently inclined to the horizontal so that bulk material can be loaded onto the deck at or near an upper loading end e.g. by discharge from an elevator or from a bucket loader, and then moved along the screening surface towards a lower discharge end while undergoing a screening action in which material smaller in size than the screening apertures is separated-out and is discharged downwardly through the deck to be received by a discharge arrangement e.g. a discharge chute or a discharge conveyor.
The screen deck may be coupled with a vibratory device which applies vibration energy to the deck in order to promote the screening action, and with the deck inclined to the horizontal, gravity action assists the movement of the bulk material over the screening surface. However, in some circumstances, the deck may be arranged substantially horizontally, so that forward movement of the bulk material is derived mainly from the way in which the vibration energy is transmitted to the deck.
Evidently, the actual residence time on the screening deck of any particular part of bulk material being screened will depend upon (a) the nature of the material (b) the slope of the deck and (c) the vibratory energy transmitted to the deck. However, the desired residence time will be a compromise between achieving (1) an acceptable rate of throughput of bulk material and (2) screening efficiency. Thus, a more steeply inclined deck can achieve a faster throughput, but with disadvantage of lowered screening efficiency in that potentially screenable material may be carried over to be discharged with non-screenable material (material greater in size than the screening apertures).
It is therefore known to provide adjustable screen decks (and so-called screen boxes), so that the angle of inclination to the horizontal of the screening surface can be adjusted according to the nature of the material to be screened e.g. dry small aggregate can be handled efficiently with a steeper deck slope than is necessary for wet soil which will require a longer residence time.
After passing through the screening apertures, the screened material may be discharged to a stockpile via a discharge conveyor. Alternatively, the screened material may fall onto a further screening deck with different size screening apertures, if it is required to obtain a further screened size range of material.
The non-screened material which is discharged from a screen deck can be conveyed to any suitable discharge point, or if required can be routed to a further screening device.
Therefore, at present, a screen deck which provides a specific screening function can discharge one portion of screened material downwardly through the screening apertures under gravity action for further handling e.g. discharge to a stockpile or to treatment by a further separate screening device at a different level; and can discharge a second portion of non-screened material from the discharge end of the deck for separate further handling e.g. discharge to a stockpile or treatment by a further and separate screening device also arranged at a different level.
Therefore, existing designs of screening plant usually have a number of screen decks arranged at different levels in order to screen material into different size ranges, and evidently the rate of output of the various screened size ranges of material will be dependent, inter alia, upon the screening area of each deck. If a large throughput of a particular size range is required, then a long deck will be provided, whereas for smaller rates of screening a shorter length of deck may be sufficient. Regardless of the length of the deck, at present entire decks are adjustable when it is desired to adjust the inclination of the deck for the purposes of varying the residence time of any particular material to be screened on the deck.
In the case of a long deck, this may have an adverse effect on screening efficiency since, with the entire screening surface of the deck inclined at the same angle, the speed of movement of the bulk material over the screening surface may vary lengthwise of the deck with consequent variation in effectiveness of the screening action. Thus, at the upper loading end of the deck, the layer of deposited material will be much thicker usually than the material as it moves along the deck towards the discharge end, with consequent variation in speed and screening efficiency. Therefore, with a long deck, the adjustment of the screening deck angle will necessarily have to be a compromise between a desired screening angle at the upper end of the deck (to give a particular desired residence time at the upper end) and the desired screening angle at other regions of the deck.
The present invention seeks to provide a two section screen deck arrangement having improved adjustability of the deck angle compared with existing screen deck designs.
According to the invention there is provided a screening device for screening bulk material and having a screen deck with a loading end for receiving a supply of bulk material and an opposite discharge end for discharging the residue of the bulk material after passage over the screen deck, and a frame on which the screen deck is mounted; in which the screen deck comprises:
a primary screening section having a loading end to receive the supply of bulk material, and arranged to carry out a preliminary screening action and to discharge the residue of the non-screened material via an opposite discharge end;
a secondary screening section having a receiving end adjacent to the discharge end of the primary section and arranged to carry out a secondary screening action and to discharge residual non-screened material via an opposite discharge end; and,
adjustment means permitting adjustment of the screening angle of one of the sections relative to the other section.
Therefore, by the invention, there is provided a screening deck having at least two sections, and with independent adjustment of the screening angle of one section relative to the other, so as to permit optimum selection of screening angles for each section.
Preferably, the primary and secondary screening sections are mounted on a common sub-frame, which is itself adjustably mounted on a main frame. Therefore, adjustment of the sub-frame can set a general screening angle for the screen deck, and then separate adjustment of the screening angle of said one section can be carried out if required, so that, for any particular screening requirements, the primary and secondary screening sections can have separate screening angles set to provide optimum screening efficiency.
If a longer screening deck is required, more than two sections (the primary and secondary sections) may be provided, and with independent adjustment of the screening angle of at least one of the sections relative to the other section(s).
In a preferred arrangement, the adjustment of the sub-frame will determine the screening angle of the secondary screening section, and the independent adjustment means will permit setting of a required screening angle of the primary section.
The screening device according to the invention may be incorporated in a static screening plant assembly. Alternatively, it may be incorporated in a mobile screening apparatus, preferably self-propelled.
Depending upon the required number of screened size ranges, there will be provision of further screening devices e.g. further screen decks, and preferably separate discharge conveyors will be provided in order to discharge, to separate stockpiles, screened material in different size ranges. The discharge conveyors are preferably foldable, so that they can take-up a suitable transport position, and can be deployed to outwardly extending discharge positions when in operation. The discharge conveyors may include tail conveyors and one or more side conveyors.
If a long multi-deck screen is required, then in a preferred development of the invention primary and secondary screen box sections are provided, which are otherwise similar to the primary and secondary screening sections referred to above.